1. Field of the Invention
The present invention relates to a bioelectrical impedance measuring method and a body composition measuring apparatus.
2. Description of the Prior Art
An electrical impedance of a living body is typically represented by a lumped constant equivalent circuit comprising an extra-cellular fluid resistance Re, an intra-cellular fluid resistance Ri, and a cell membrane capacitance Cm, as shown in FIG. 1. Practically, plural cells making up the living body are respectively represented by individual circuits having different constants due to their different shapes and characteristics. Thus, in the living body as an aggregation of such cells, its vector impedance locus does not show a half circle at variance with the case of measuring the lumped constant equivalent circuit, but shows a circular arc given in the Cole-Cole model.
Thus, the electrical impedance of the living body is generally represented by a circular arc-like locus shown in FIG. 2. In FIG. 2, x-axis represents a resistance component of the impedance, while y-axis represents a reactance component of the impedance. Since the reactance component of the bioelectrical impedance shows a negative value due to its capacitive property, the vector locus of the bioelectrical impedance is plotted on the underside of the real axis as shown in FIG. 2.
Referring to FIG. 3, R0, Rinf, and Zc respectively indicate a resistance at 0 frequency, a resistance at infinite frequency and a bioelectrical impedance value at frequency Fc. As to R0 and Rinf, they have only a resistance component because their reactance value is zero. At the frequency Fc, an absolute value of the reactance component reaches its maximum, and Zc is a bioelectrical impedance value at this frequency. As used herein, the frequency where the absolute value of the reactance component reaches its maximum is referred as to a characteristic frequency. Each body composition, such as a total body water, an intra-cellular water, an extra-cellular water, and a fat-free mass, is derived from the above values or approximate values thereof.
In a conventional method for determining the bioelectrical vector impedance locus based on bioelectrical impedances measured at a plurality of frequencies, the bioelectrical impedance is firstly measured in the range from a low frequency to a high frequency (i.e. from several kHz to about 1 MHz). Then, the aforementioned circular arc-like vector locus is derived from the measured data to calculate the above parameters.
Generally, the impedance vector measured by the conventional method is not provided in the form of a circular arc shown by a solid line in FIG. 2, but is represented in a curve-like locus shown by a dotted line in FIG. 2. This is supposedly resulted from a time lag in a signal transmission system which is influenced by both lengths of a bioelectrical impedance measuring cable and a measuring object. Practically, the least square approximation method would be applied to correct such an error and to make the vector impedance locus approximate to the circular arc. Making an approximate calculation requires multiplicity of iterative operations and thereby demands a high-speed arithmetic unit and a peripheral device thereof.
Thus, the conventional bioelectrical impedance measuring apparatus needs to employ the high speed arithmetic unit and the associated peripheral device. In addition, since it takes a long time for the measurement, a patient is forced to keep a specified posture for a long time. This has applied a certain burden to the patient.
An object of the present invention is to provide an improved bioelectrical impedance measuring method and a body composition measuring apparatus, which is capable of solving the problems of the prior art described above.
According to an aspect of the present invention, there is provided a bioelectrical impedance measuring method for measuring a bioelectrical impedance of a patient by applying an alternating current to a body of the patient, said bioelectrical impedance measuring method comprising the steps of:
determining a first bioelectrical impedance value by a measurement using the alternating current having a first frequency;
determining a second bioelectrical impedance value by a measurement using the alternating current having a second frequency;
determining a third bioelectrical impedance value by a measurement using the alternating current having a third frequency; and
deriving a vector impedance locus from only the first, second and third bioelectrical impedance values to determine bioelectrical impedance values at 0 frequency and at an infinite frequency.
According to an embodiment of the present invention, all the first, second and third frequencies may be in the range of 1 kHz to 100 kHz.
According to another aspect of the present invention, there is provided a body composition measuring apparatus for measuring a bioelectrical impedance of a patient by applying an alternating current to a body of the patient based on the bioelectrical impedance method, said body composition measuring apparatus comprising:
an alternating current generating device capable of generating at least three kinds of alternating currents with different frequencies;
a measuring device which determines a first bioelectrical impedance value, a second bioelectrical impedance value and a third bioelectrical impedance value based on measurements using the alternating current having a first frequency, the alternating current having a second frequency and the alternating current having a third frequency respectively, among the alternating currents generated by the alternating current generating device;
an arithmetic device which derives a vector impedance locus from only the determined first, second and third bioelectrical impedance values to determine bioelectrical impedance values at 0 frequency and at an infinite frequency; and
a judging device which judges the body composition of the patient based on the bioelectrical impedance values determined by the arithmetic device.
According to another embodiment of the present invention, said body composition measuring apparatus further comprises:
an input device which sets a personal parameter including a body weight of the patient; and
an indicating device which indicates information regarding the body composition judged by said judging device, wherein said judging device takes the personal parameter input by the input device into account when judging the body composition of the patient.
According to another embodiment of the present invention, the body composition measuring apparatus may further comprise:
a body weight measuring device which measures the body weight of the patient;
an input device which sets a personal parameter other than the body weight of the patient; and
an indicating device which indicates information regarding the body composition of the patient judged by said judging device, wherein said judging device takes the body weight measured by the body weight measuring device and the personal parameter input by the input device into account when judging the body composition of the patient.
According to another embodiment of the present invention, in the body composition measuring apparatus, all the first, second and third frequencies may be in the range of 1 kHz to 100 kHz.
According to another embodiment of the present invention, in the body composition measuring apparatus, the body composition may be at least one of an extra-cellular water, an intra-cellular water, a total body water, a fat-free mass, and a body fat mass.
The present invention will now be described in further detail with regard to preferred embodiments as illustrated in the accompanying drawings.